Consecutive Fertilization-Promoted Soil Nutrient Availability and Altered Rhizosphere Bacterial and Bulk Fungal Community Composition

Fertilization is an important measure to quickly supplement the soil nutrients required for plantation productivity. However, the response patterns of the microbial community and functional taxa in Larix plantation root, rhizosphere, and bulk soil to short-term and consecutive fertilization have rarely been reported. In this study, we assessed Larix root, rhizosphere, and bulk soil microbial community on days 0, 5, 15, and 30 after the first inorganic fertilization and after three consecutive years of fertilization. The bacterial 16S and fungal ITS high-throughput sequencing technology were used to monitor changes in microbial community composition and potential functional groups, as well as changes in soil nutrient content and enzyme activity to evaluate the status of plantation soil productivity. Consecutive fertilization treatment significantly increased the available nitrogen, phosphorus, and potassium (NPK) content and soil enzyme activity. The nonmetric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM) results showed that there were significant differences in microbial community composition in root samples, rhizosphere soil, and bulk soil samples. The dominant microbial taxa were different between root and soil microbial community composition. Consecutive fertilization treatment had little effect on endophytic microbial community but significantly increased the abundance of Gaiellales in rhizosphere soil and Mortierella in bulk soil. The redundancy analysis (RDA) and co-occurrence network analyses showed that Gaiellales and Mortierellales had significant positive correlations with soil nutrient content and enzyme activity. The fungal functional group compositions were significantly affected by consecutive fertilization treatment and the proportions of ectomycorrhizal and saprotroph significantly decreased, but the proportion of endophyte significantly increased in bulk soil samples. Our results suggested that consecutive fertilization may promote soil nutrient availability by increasing the abundance of Gaiellales and Mortierella. Consecutive fertilization maintained the balance of the soil microbiota under Larix plantation and had a positive effect on promoting soil nutrient availability. This study provided a theoretical basis for consecutive fertilization to promote soil nutrient availability through specific microbial groups.